An Embedded Auto-Calibrated Offset Current Compensation Technique for PPG/fNIRS System

Optical sensors play a critical role in biomedical monitoring systems, enabling non-invasive measurement of physiological parameters. However, these systems face a persistent challenge: the small amplitude of physiological signals is often overshadowed by a dominant DC offset current. This offset combines the dark current, ambient light, and reflections from fat and skull. This limits the amplification of signals and degrades the sensitivity of PPG/fNIRS systems. This letter presents a mixed-signal auto-calibrated offset compensation technique using dual discrete loops for PPG and fNIRS systems. Two specialized prototypes -a dedicated PPG system and an fNIRS system -were validated using a 3.3 V supply. The results show that the proposed system can effectively compensate for the offset currents within a range of 1lA to 100 lA. We have recorded PPG and fNIRS signals and performed breathing experiments to validate the results. The results show that the proposed system is able to record the signal accurately.